1. Field of Invention
The invention relates to treatment methods for a flexible multi-layered electrostatographic imaging member, such as web stock. In particular, the invention relates to an improved stress relief treatment process for flexible imaging member web stock to impart mechanical service life extension.
2. Description of Related Art
Flexible electrostatographic imaging members are well known in the electrostatographic marking art. Typical flexible electrostatographic imaging members include, for example, (1) electrophotographic imaging members (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems and (2) electroreceptors, such as ionographic imaging members for electrographic imaging systems. The flexible electrostatographic imaging members can be in the form of seamless or seamed belts. Typical electrophotographic imaging member belts comprise a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating applied to the opposite side of the supporting substrate layer to induce flatness. Electrographic imaging member belts, however, typically have a more simple material structure, including a dielectric imaging layer on one side of a supporting substrate and an anticurl back coating on the opposite side of the substrate. While the scope of embodiments covers an improved preparation process for flexible electrostatographic imaging members producing a crack resistance enhanced outer top imaging layer, the following discussion will focus only on processing of flexible electrophotographic imaging members for simplicity.
Electrophotographic flexible imaging members typically comprise a photoconductive layer, which can include a single layer or composite layers. Since typical electrophotographic imaging members can exhibit undesirable upward imaging member curling, the anticurl back coating brings each imaging member to at least a desired flatness.
One type of composite photoconductive layer used in electrophotography, illustrated in U.S. Pat. No. 4,265,990, for example, the disclosure of which is hereby incorporated by reference, has at least two electrically operative layers. One layer comprises a photoconductive layer that can photogenerate holes and inject the holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. The supporting electrode can still function as an anode when the charge transport layer is sandwiched between the supporting electrode and the photoconductive layer. The charge transport layer in this case must be able to support the injection of photogenerated electrons from the photoconductive layer and to transport the electrons through the charge transport layer. Photosensitive members having at least two electrically operative layers can provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member, such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have created stringent requirements including narrow operating limits on photoreceptors. For flexible electrophotographic imaging members having a belt configuration, the numerous layers found in modem photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor belt that has been employed as a belt in negatively charging electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor belt can also comprise additional layers, such as an anticurl back coating to balance curl and provide the desired belt flatness.
In a machine service environment, a flexible multilayered photoreceptor belt, mounted on a belt supporting module that includes a number of support rollers, is generally exposed to repetitive electrophotographic image cycling, which subjects the outer-most charge transport layer to mechanical fatigue as the imaging member belt bends and flexes over the belt drive roller and all other belt module support rollers. The outer-most layer also experiences bending strain as the backside of the belt makes sliding and/or bending contact above each backer bar's curving surface. This repetitive action of belt cycling leads to a gradual deterioration in the physical/mechanical integrity of the exposed outer charge transport layer, leading to premature onset of fatigue charge transport layer cracking. The cracks developed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print defects, which thereby adversely affect the image quality on the receiving paper. In essence, the appearance of charge transport cracking cuts short the imaging member belt's intended functional life.
When a production web stock consisting of several thousand feet of coated multilayered photoreceptor is obtained after finishing the charge transport layer coating/drying process, it is seen to spontaneously curl upwardly. Hence, the anticurl back coating is applied to the backside of the substrate support, opposite to the side having the charge transport layer, to counteract the curl and render the photoreceptor web stock flatness. The exhibition of upward photoreceptor curling after completion of charge transport layer coating results from thermal contraction mismatch between the applied charge transport layer and the substrate support under the conditions of elevated temperature heating/drying the wet coating and eventual cooling down to room ambient temperature. Since the charge transport layer in a typical photoreceptor device has a coefficient of thermal contraction approximately 2 to 5 times larger than that of the substrate support, upon cooling down to room ambient, greater dimensional contraction occurs in the charge transport layer than in the substrate support. This yields the upward photoreceptor curling of the web stock.
Although, in a typical photoreceptor belt, it is necessary to apply an anticurl back coating to complete a typical photoreceptor web stock material package having the desired flatness, nonetheless the application of the anticurl back coating onto the backside of the substrate support (for counter-acting the upward curling and render photoreceptor web stock flatness) has caused the charge transport layer to instantaneously build-in an internal tension strain of from about 0.15% to about 0.35% in its coating material matrix. After converting the production web stock into seamed photoreceptor belts, the internal built-in strain in the charge transport layer is then cumulatively added to each photoreceptor bending induced strain as the belt flexes over a variety of belt module support rollers during photoreceptor belt dynamic cyclic function in a machine. The consequence of this cumulative strain effect has been found to cause the acceleration and early onset of photoreceptor belt fatigue charge transport layer cracking problem. Moreover, the cumulative charge transport layer strain has also been identified as the origin of the formation of bands of charge transport layer cracking when the photoreceptor belt is parked over the belt support module during periods of machine idling or overnight and weekend shut-off time, as the belt is under constant airborne chemical vapor and contaminants exposure. The bands of charge transport layer cracking are formed at the sites corresponding to photoreceptor belt bending over each of the belt supporting rollers. The crack intensity is also seen to be most pronounced for the band at the belt segment bent and parked directly over the smallest roller, since according to the fundamentals of material mechanics, the smaller the roller diameter the belt segment is bent over, the greater is the bending strain is induced in the charge transport layer surface.
Thus, there is a need for a method of fabrication of improved flexible seamed photoreceptor belts, having a charge transport layer with little or no built-in internal tension and reduced bending strain as the belts flex during machine function or during static bent belt parking over the belt module support rollers under the periods of machine idling and shut-off. Such belts will enjoy extended mechanical functioning life and effect the suppression of premature onset of charge transport layer cracking problem as well.
U.S. Patent Publication No. 20030067097, U.S. patent application Ser. No. 10/385,409, and U.S. Pat. Nos. 5,606,396, 5,089,369, 5,167,987, and 4,983,481, the disclosures of which are hereby incorporated by reference, represent prior efforts toward alleviating the problems discussed above. Although these efforts were successful to a point, however, resolution of one problem had often been found to create new ones. For example, the processing disclosed in the above-described applications have been found to introduce a new problem of copy streak printout defect issue.
Thus, there is a continued need to improve the methodology for cost effectual production of flexible imaging members, particularly through innovative processing treatment approaches that effect charge transport layer internal tension strain reduction or elimination, as well as reduction the bending/flexing strain over belt module support rollers, in multilayered electrophotographic imaging member web stocks to yield mechanically robust imaging member belts to meet future machine imaging member belt life extension need.